FR-A-2 481 755 describes and shows a pressurized fluid mechanism, such as a reversible hydraulic motor or pump having at least two operating cylinder capacities, and comprising: a stator; a rotor rotatable relative to the stator about a rotation axis; main fluid inlet and outlet chambers, one adapted to communicate selectively with a source of high-pressure fluid and the other adapted to communicate selectively with a low-pressure return; an undulating cam including a plurality of undulations divided into two groups of first undulations and second undulations, respectively; a plurality of cylinders inside each of which slides at least one piston defining a working chamber inside said cylinder; an internal distributor for distributing fluid to said working chambers including first secondary inlet and outlet chambers for distributing fluid to the working chambers in the cylinders facing the undulations of the first group of undulations and second secondary inlet and outlet chambers for distributing fluid to the working chambers in the cylinders facing the undulations of the second group of undulations; and a capacity selector device which in a "large capacity" first configuration connects in parallel firstly the first and second secondary inlet chambers and secondly the first and second secondary outlet chambers and in a "small capacity" second configuration isolates the second secondary inlet and outlet chambers from whichever of the main inlet and outlet chambers contains the fluid at the higher pressure and establishes communication of said second secondary inlet and outlet chambers with each other and with whichever of the main inlet and outlet chambers contains the fluid at the lower pressure.
The operation of a mechanism of this kind is satisfactory but its design requires the manufacture of specialized, and therefore relatively costly, components disposed at specific locations, with the result that its overall axial and radial dimensions are relatively large. One of the specialized components is a shuttle valve contained in a capacity selector slide valve of that prior art mechanism.